
In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration. The concentration can refer to any kind of chemical mixture, but most frequently refers to solutes and solvents in solutions. The molar (amount) concentration has variants, such as normal concentration and osmotic concentration. Dilution is reduction of concentration, e.g. by adding solvent to a solution. The verb to concentrate means to increase concentration, the opposite of dilute.
Etymology
Concentration-, concentratio, action or an act of coming together at a single place, bringing to a common center, was used in post-classical Latin in 1550 or earlier, similar terms attested in Italian (1589), Spanish (1589), English (1606), French (1632).
Qualitative description
Often in informal, non-technical language, concentration is described in a qualitative way, through the use of adjectives such as "dilute" for solutions of relatively low concentration and "concentrated" for solutions of relatively high concentration. To concentrate a solution, one must add more solute (for example, alcohol), or reduce the amount of solvent (for example, water). By contrast, to dilute a solution, one must add more solvent, or reduce the amount of solute. Unless two substances are miscible, there exists a concentration at which no further solute will dissolve in a solution. At this point, the solution is said to be saturated. If additional solute is added to a saturated solution, it will not dissolve, except in certain circumstances, when supersaturation may occur. Instead, phase separation will occur, leading to coexisting phases, either completely separated or mixed as a suspension. The point of saturation depends on many variables, such as ambient temperature and the precise chemical nature of the solvent and solute.
Concentrations are often called levels, reflecting the mental schema of levels on the vertical axis of a graph, which can be high or low (for example, "high serum levels of bilirubin" are concentrations of bilirubin in the blood serum that are greater than normal).
Quantitative notation
There are four quantities that describe concentration:
Mass concentration
The mass concentration is defined as the mass of a constituent
divided by the volume of the mixture
:
The SI unit is kg/m3 (equal to g/L).
Molar concentration
The molar concentration is defined as the amount of a constituent
(in moles) divided by the volume of the mixture
:
The SI unit is mol/m3. However, more commonly the unit mol/L (= mol/dm3) is used.
Number concentration
The number concentration is defined as the number of entities of a constituent
in a mixture divided by the volume of the mixture
:
The SI unit is 1/m3.
Volume concentration
The volume concentration (not to be confused with volume fraction) is defined as the volume of a constituent
divided by the volume of the mixture
:
Being dimensionless, it is expressed as a number, e.g., 0.18 or 18%.
There seems to be no standard notation in the English literature. The letter used here is normative in German literature (see Volumenkonzentration).
Related quantities
Several other quantities can be used to describe the composition of a mixture. These should not be called concentrations.
Normality
Normality is defined as the molar concentration divided by an equivalence factor
. Since the definition of the equivalence factor depends on context (which reaction is being studied), the International Union of Pure and Applied Chemistry and National Institute of Standards and Technology discourage the use of normality.
Molality
The molality of a solution is defined as the amount of a constituent
(in moles) divided by the mass of the solvent
(not the mass of the solution):
The SI unit for molality is mol/kg.
Mole fraction
The mole fraction is defined as the amount of a constituent
(in moles) divided by the total amount of all constituents in a mixture
:
The SI unit is mol/mol. However, the deprecated parts-per notation is often used to describe small mole fractions.
Mole ratio
The mole ratio is defined as the amount of a constituent
divided by the total amount of all other constituents in a mixture:
If is much smaller than
, the mole ratio is almost identical to the mole fraction.
The SI unit is mol/mol. However, the deprecated parts-per notation is often used to describe small mole ratios.
Mass fraction
The mass fraction is the fraction of one substance with mass
to the mass of the total mixture
, defined as:
The SI unit is kg/kg. However, the deprecated parts-per notation is often used to describe small mass fractions.
Mass ratio
The mass ratio is defined as the mass of a constituent
divided by the total mass of all other constituents in a mixture:
If is much smaller than
, the mass ratio is almost identical to the mass fraction.
The SI unit is kg/kg. However, the deprecated parts-per notation is often used to describe small mass ratios.
Dependence on volume and temperature
Concentration depends on the variation of the volume of the solution with temperature, due mainly to thermal expansion.
Table of concentrations and related quantities
Concentration type | Symbol | Definition | SI unit | other unit(s) |
---|---|---|---|---|
mass concentration | kg/m3 | g/100mL (= g/dL) | ||
molar concentration | mol/m3 | M (= mol/L) | ||
number concentration | 1/m3 | 1/cm3 | ||
volume concentration | m3/m3 | |||
Related quantities | Symbol | Definition | SI unit | other unit(s) |
normality | mol/m3 | M (= mol/L) | ||
molality | mol/kg | m | ||
mole fraction | mol/mol | ppm, ppb, ppt | ||
mole ratio | mol/mol | ppm, ppb, ppt | ||
mass fraction | kg/kg | ppm, ppb, ppt | ||
mass ratio | kg/kg | ppm, ppb, ppt | ||
volume fraction | m3/m3 | ppm, ppb, ppt |
See also
- Dilution ratio – Change in concentration when mixing two liquids
- Dose concentration – Ratio of part of a mixture to the whole
- Serial dilution – Step-wise dilution of a substance in solution
- Wine/water mixing problem
- Standard state § Solutes
References
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "concentration". doi:10.1351/goldbook.C01222
- "concentration". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "volume fraction". doi:10.1351/goldbook.V06643
External links
Media related to Concentration (chemistry) at Wikimedia Commons
In chemistry concentration is the abundance of a constituent divided by the total volume of a mixture Several types of mathematical description can be distinguished mass concentration molar concentration number concentration and volume concentration The concentration can refer to any kind of chemical mixture but most frequently refers to solutes and solvents in solutions The molar amount concentration has variants such as normal concentration and osmotic concentration Dilution is reduction of concentration e g by adding solvent to a solution The verb to concentrate means to increase concentration the opposite of dilute EtymologyConcentration concentratio action or an act of coming together at a single place bringing to a common center was used in post classical Latin in 1550 or earlier similar terms attested in Italian 1589 Spanish 1589 English 1606 French 1632 Qualitative descriptionThese glasses containing red dye demonstrate qualitative changes in concentration The solutions on the left are more dilute compared to the more concentrated solutions on the right Often in informal non technical language concentration is described in a qualitative way through the use of adjectives such as dilute for solutions of relatively low concentration and concentrated for solutions of relatively high concentration To concentrate a solution one must add more solute for example alcohol or reduce the amount of solvent for example water By contrast to dilute a solution one must add more solvent or reduce the amount of solute Unless two substances are miscible there exists a concentration at which no further solute will dissolve in a solution At this point the solution is said to be saturated If additional solute is added to a saturated solution it will not dissolve except in certain circumstances when supersaturation may occur Instead phase separation will occur leading to coexisting phases either completely separated or mixed as a suspension The point of saturation depends on many variables such as ambient temperature and the precise chemical nature of the solvent and solute Concentrations are often called levels reflecting the mental schema of levels on the vertical axis of a graph which can be high or low for example high serum levels of bilirubin are concentrations of bilirubin in the blood serum that are greater than normal Quantitative notationThere are four quantities that describe concentration Mass concentration The mass concentration ri displaystyle rho i is defined as the mass of a constituent mi displaystyle m i divided by the volume of the mixture V displaystyle V ri miV displaystyle rho i frac m i V The SI unit is kg m3 equal to g L Molar concentration The molar concentration ci displaystyle c i is defined as the amount of a constituent ni displaystyle n i in moles divided by the volume of the mixture V displaystyle V ci niV displaystyle c i frac n i V The SI unit is mol m3 However more commonly the unit mol L mol dm3 is used Number concentration The number concentration Ci displaystyle C i is defined as the number of entities of a constituent Ni displaystyle N i in a mixture divided by the volume of the mixture V displaystyle V Ci NiV displaystyle C i frac N i V The SI unit is 1 m3 Volume concentration The volume concentration si displaystyle sigma i not to be confused with volume fraction is defined as the volume of a constituent Vi displaystyle V i divided by the volume of the mixture V displaystyle V si ViV displaystyle sigma i frac V i V Being dimensionless it is expressed as a number e g 0 18 or 18 There seems to be no standard notation in the English literature The letter si displaystyle sigma i used here is normative in German literature see Volumenkonzentration Related quantitiesSeveral other quantities can be used to describe the composition of a mixture These should not be called concentrations Normality Normality is defined as the molar concentration ci displaystyle c i divided by an equivalence factor feq displaystyle f mathrm eq Since the definition of the equivalence factor depends on context which reaction is being studied the International Union of Pure and Applied Chemistry and National Institute of Standards and Technology discourage the use of normality Molality The molality of a solution bi displaystyle b i is defined as the amount of a constituent ni displaystyle n i in moles divided by the mass of the solvent msolvent displaystyle m mathrm solvent not the mass of the solution bi nimsolvent displaystyle b i frac n i m mathrm solvent The SI unit for molality is mol kg Mole fraction The mole fraction xi displaystyle x i is defined as the amount of a constituent ni displaystyle n i in moles divided by the total amount of all constituents in a mixture ntot displaystyle n mathrm tot xi nintot displaystyle x i frac n i n mathrm tot The SI unit is mol mol However the deprecated parts per notation is often used to describe small mole fractions Mole ratio The mole ratio ri displaystyle r i is defined as the amount of a constituent ni displaystyle n i divided by the total amount of all other constituents in a mixture ri nintot ni displaystyle r i frac n i n mathrm tot n i If ni displaystyle n i is much smaller than ntot displaystyle n mathrm tot the mole ratio is almost identical to the mole fraction The SI unit is mol mol However the deprecated parts per notation is often used to describe small mole ratios Mass fraction The mass fraction wi displaystyle w i is the fraction of one substance with mass mi displaystyle m i to the mass of the total mixture mtot displaystyle m mathrm tot defined as wi mimtot displaystyle w i frac m i m mathrm tot The SI unit is kg kg However the deprecated parts per notation is often used to describe small mass fractions Mass ratio The mass ratio zi displaystyle zeta i is defined as the mass of a constituent mi displaystyle m i divided by the total mass of all other constituents in a mixture zi mimtot mi displaystyle zeta i frac m i m mathrm tot m i If mi displaystyle m i is much smaller than mtot displaystyle m mathrm tot the mass ratio is almost identical to the mass fraction The SI unit is kg kg However the deprecated parts per notation is often used to describe small mass ratios Dependence on volume and temperatureConcentration depends on the variation of the volume of the solution with temperature due mainly to thermal expansion Table of concentrations and related quantitiesConcentration type Symbol Definition SI unit other unit s mass concentration ri displaystyle rho i or gi displaystyle gamma i mi V displaystyle m i V kg m3 g 100mL g dL molar concentration ci displaystyle c i ni V displaystyle n i V mol m3 M mol L number concentration Ci displaystyle C i Ni V displaystyle N i V 1 m3 1 cm3volume concentration si displaystyle sigma i Vi V displaystyle V i V m3 m3Related quantities Symbol Definition SI unit other unit s normality ci feq displaystyle c i f mathrm eq mol m3 M mol L molality bi displaystyle b i ni msolvent displaystyle n i m mathrm solvent mol kg mmole fraction xi displaystyle x i ni ntot displaystyle n i n mathrm tot mol mol ppm ppb pptmole ratio ri displaystyle r i ni ntot ni displaystyle n i n mathrm tot n i mol mol ppm ppb pptmass fraction wi displaystyle w i mi mtot displaystyle m i m mathrm tot kg kg ppm ppb pptmass ratio zi displaystyle zeta i mi mtot mi displaystyle m i m mathrm tot m i kg kg ppm ppb pptvolume fraction ϕi displaystyle phi i Vi jVj displaystyle V i sum j V j m3 m3 ppm ppb pptSee alsoDilution ratio Change in concentration when mixing two liquids Dose concentration Ratio of part of a mixture to the wholePages displaying short descriptions of redirect targets Serial dilution Step wise dilution of a substance in solution Wine water mixing problem Standard state SolutesReferencesIUPAC Compendium of Chemical Terminology 2nd ed the Gold Book 1997 Online corrected version 2006 concentration doi 10 1351 goldbook C01222 concentration Oxford English Dictionary Online ed Oxford University Press Subscription or participating institution membership required IUPAC Compendium of Chemical Terminology 2nd ed the Gold Book 1997 Online corrected version 2006 volume fraction doi 10 1351 goldbook V06643External linksMedia related to Concentration chemistry at Wikimedia Commons